As integrated circuits are made smaller, low resistivity materials are needed as interconnects. Metal silicides such as tungsten (W), titanium (Ti), molybdenum (Mo) and cobalt (Co) polycide are very attractive candidates. Titanium polycide is especially attractive since it indicates lowest resistivity.
Self-aligned silicide (or salicide) processes are presently used for lowering the resistivity of transfer gates and n- or p-type diffused layers. See e.g., Science Forum Corp., "Ti SALICIDE Process," VLSI Process Data Book, p322. But as gate widths become narrower, very shallow junctions will be necessary to avoid short channel effects. Unfortunately, it is difficult to apply conventional titanium salicide processes to devices with shallow junctions since the depth from the titanium suicide (TiSi.sub.2) interface to the p-n (or n-p) junction is marginal.
It is known that among refractory metal suicides, TiSi.sub.2 is considered to be an optimal choice for applications such as contacts and interconnects on silicon MOS devices. The thin-film reaction of titanium on silicon often results in the formation of TiSi.sub.2. The two different crystal structures of TiSi.sub.2 most often observed in thin film reactions are the C49 and C54 types. The C49 structure is base-centered orthorhombic and the C54 structure is face-centered orthorhombic. The C54 phase is the only TiSi.sub.2 phase which occurs in the binary-phase diagram, and therefore the C49 phase is considered to be metastable. In general, see Jeon et al., "Morphology and phase stability of TiSi.sub.2 on Si," J. Appl. Phy. 71(9), May 1, 1992, pp4269-76.
During fabrication, the metastable C49 structure is formed at a relatively low temperature, for example at about 500.degree. C., while the stable C54 structure is formed at higher temperatures of about 700.degree. C. During formation of a titanium silicide layer, the initial nucleation will be to the metastable C49 phase rather than the C54 phase due to a lower free-energy barrier to nucleation. Unfortunately, the transition from the C49 phase to the C54 phase is difficult because C54 has high packing density compared with C49. This density difference causes a reduction in volume during the anneal process. However, a thin TiSi.sub.2 on a rigid surface, such as a silicon substrate, cannot shrink freely since the adhesive bond before any phase transition restricts reduction of volume and causes a tensile stress. If the volume of the C54 phase TiSi.sub.2 is expanded by 1.08 times (if a tensile stress of more than 380 MPa is applied), then the total energy of C49 TiSi.sub.2 will be less than that of C54 TiSi.sub.2. See "Theoretical study of stress induced C54-to-C49 transition of TiSi.sub.2," Extended Abstracts of the 56th Annual Meeting, 1995, The Japan Society of Applied Physics.
It is known to fabricate a titanium silicide layer using two annealing steps. For example, U.S. Pat. No. 5,043,300 (which issued Aug. 27, 1991 and is incorporated by reference herein) teaches depositing a titanium layer on a cleaned semiconductor wafer. The wafer is then moved from a vacuum deposition chamber to an annealing chamber being careful not to expose it to oxygen-bearing gases. Within the annealing chamber, the wafer is annealed in a nitrogen-bearing atmosphere for about 20 to 60 seconds at a temperature from about 500.degree. C. to about 695.degree. C. This process step forms a titanium silicide layer and a layer of titanium nitride over the suicide. In addition, the titanium which had been deposited over silicon oxide surfaces is reacted to form titanium nitride. The wafer temperature is then raised to between about 800.degree. C. and 900.degree. C. to convert the titanium suicide to a stable phase. The wafer can then be etched to remove titanium nitride.
Two papers were presented at the August 1995 meeting of the Japan Society of Applied Physics in Kanazawa, Japan. In "Theoretical Study on Stress-Induced C54 to C49 Phase Transition of TiSi.sub.2," Ohfuti et al. studied theoretically how stress affects a phase transition of TiSi.sub.2. In this paper, the authors include a figure which shows the volume dependence of the total energy of C49 and C54 structure d TiSi.sub.2. The volume is normalized by th at of non-stressed C54. The total energy of C54 becomes higher than that of C49 when the volume is expanded over 1.08. This value corresponds to the tensile stress of 380 Mpa, obtained by differentiating the energy versus volume relation. In this paper, the authors conclude that the phase transition from C49 to C54 may not take place under the tensile stress over 380 Mpa.
At t his conference, Kawamura et al. also presented a paper entitled "Stress Effects on the C49-C54 Phase Transition of TiSi.sub.2." In the formation of TiSi.sub.2, the so-called two-step annealing process is generally used. In the first step, Ti/Si bilayer (patterned or non-patterned) is annealed at a low temperature to form C49 phase TiSi.sub.2. Then in the second step, C49 phase TiSi.sub.2 is annealed at higher temperature to transition to C54 phase. The authors of this paper tried applying compressive stress to enhance the C49 to C54 phase transition by depositing a titanium layer on the backside of the Si wafer. They deposited Ti layers on both front a nd back sides and then annealed the wafer at 600.degree. C. for fifteen minutes in an argon atmosphere to form C49 TiSi.sub.2 on both sides. After that, they annealed the wafer at 650.degree. C. for 30 seconds in argon to form C54 TiSi.sub.2.
The paper included a figure which showed how the sheet resistivity of the TiSi.sub.2 layer of the frontside decreased as a function of the annealing time (a number of annealing cycles at 650.degree. C. for 30 seconds each). The Ti thickness of the backside was varied as another parameter. The results show that the sheet resistivity of TiSi.sub.2 decreases rapidly as the backside Ti thickness increases. The authors measured the stress applied to the TiSi.sub.2 layer of the frontside and found that a compressive stress was applied when there was the backside TiSi.sub.2.